1. Field of the Invention
The present invention relates to the field of non-volatile memory devices. More particularly, the invention relates to a multi-bit flash electrically erasable programmable read only memory (EEPROM) cell with a bitline.
2. Discussion of Related Art
Memory devices for non-volatile storage of information are currently in widespread use today, being used in a myriad of applications. A few examples of non-volatile semiconductor memory include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM) and flash EEPROM.
Semiconductor EEPROM devices involve more complex processing and testing procedures than ROM, but have the advantage of electrical programming and erasing. Using EEPROM devices in circuitry permits in-circuit erasing and reprogramming of the device, a feat not possible with conventional EPROM memory. Flash EEPROMs are similar to EEPROMs in that memory cells can be programmed (i.e., written) and erased electrically but with the additional ability of erasing all memory cells at once, hence the term flash EEPROM.
An example of a single transistor Oxide-Nitrogen-Oxide (ONO) EEPROM device is disclosed in the technical article entitled "A True Single-Transistor Oxide-Nitride-Oxide EEPROM Device," T. Y. Chan, K. K. Young and Chenming Hu, IEEE Electron Device Letters, March 1987. The memory cell is programmed by hot electron injection and the injected charges are stored in the oxide-nitride-oxide (ONO) layer of the device. Other examples of ONO EEPROM devices are disclosed in U.S. Pat. Nos. 5,635,415; 5,768,192 and PCT patent application publication WO 99/07000, the contents of each reference are hereby incorporated herein by reference.
In the case of known NROM devices, such as schematically shown in FIG. 1, an NROM cell 100 included a grid of polygates or word lines 102 and buried bitlines 104. The bitlines 104 were formed in the N+ region of the substrate so that a higher density of bitlines can be formed that region versus when the bitlines were formed in a metal layer. Select transistors 106 were required to be placed every N or N/2 polygates 102, where N is the number of polygates between contacts 108. This in the past has required a select transistor 106 being required every 16 or 32 cells in order to reduce the bitline to cell resistance. The bitline resistance in the N+ region limits the number of cells between select transistors.
In the case of flash memory cells with a stacked gate, contacts associated with the cell must be spaced from the polysilicon of the gate. As feature sizes are reduced according to integrated circuit processes, smaller dimensions are required to achieve higher packing densities. Generally, contacts must be spaced apart from the stacked gate so alignment errors do not result in a shorting of the stacked gate with the source contact or the drain contact. The spacing between the contact and gate contributes to the overall size of the flash memory cell.